Convert CONFIG_L2_CACHE to Kconfig

This converts the following to Kconfig:
   CONFIG_L2_CACHE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/socrates_defconfig b/configs/socrates_defconfig
index 4cc50d3..46be662 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SYS_INIT_RAM_LOCK=y
 # CONFIG_CMD_ERRATA is not set
 CONFIG_TARGET_SOCRATES=y
+CONFIG_L2_CACHE=y
 CONFIG_ENABLE_36BIT_PHYS=y
 CONFIG_SYS_MONITOR_LEN=393216
 CONFIG_FIT=y